Mold release technique for solid propellant casting tooling

ABSTRACT

Tooling for solid propellant casting consisting essentially of a shaped, filled polymer or copolymer article having an outer layer of cured polyvinyl butyral. The casting tooling is used extensively throughout the solid propellant industry to mold or form the solid propellant grain of rocket motors. The cured polyvinyl butyral layer provides improved mold release properties to the tooling.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

This invention relates to tooling which is employed in the casting ofsolid rocket propellants.

A solid propellant rocket generally employs a rocket motor case,insulation, a liner composition, and a solid propellant grain in theorder listed as viewed from the outer motor case to the solid propellantgrain contained therein. The functions of each of the components of asolid propellant rocket motor are well defined in the art.

The solid propellant grain may be processed by extrusion or castingtechniques. The majority of modern composite propellants for rocketmotors are cast directly into the case as a cross-linkable mixture andcured in place.

The thrust-time characteristic of a solid propellant rocket can becontrolled by the geometric shape of the grain. Neutral burning grainsmaintain a constant surface during burning and produce a constantthrust. Progressive burning grains increase in surface during burningand produce an increasing thrust with time. Regressive burning grainsdecrease in surface during burning and produce a decreasing thrust withtime.

The geometric shape of a grain is generally understood to be the shapeof the internal perforation or perforations. Internally perforated,outwardly burning grains are superior to end burning or external burninggrains because the wall area of the motor case is protected from the hotgas generated by combustion.

The perforation in a cast solid propellant grain is produced by castingthe cross-linkable mixture around shaped casting tooling, curing themixture and withdrawing the tooling from the cured solid propellant.Glass-filled polytetrafluoroethylene casting tooling is used extensivelythroughout the solid propellant industry to mold or form the solidpropellant grain of rocket motors. From both the safety and designstandpoints, it is an ideal material of construction. It is a relativelysoft material with a low compressive strength, yet is dimensionallystable. Unfortunately, the mold release properties of this material arereduced as glass filler is added. The mold release properties alsodegrade with repeated use, because mobile propellant species migrateinto the porous glass-filled polymer substrate. Conventional moldrelease agents are ineffective with this tooling since they are notimpervious and allow species migration into the glass-filled polymersubstrate. Conventional cleaning methods are also ineffective since theydo not remove subsurface contamination. Thus, repeated usage ofglass-filled polytetrafluoroethylene casting tooling results in removaldifficulties and damage to the propellant grain in the form of surfacetears.

Accordingly, it is an object of this invention to provide a method formodifying solid propellant casting tooling to provide improved moldrelease.

It is another object of this invention to provide improved solidpropellant casting tooling.

Other objects, aspects and advantages of the present invention willbecome apparent to those skilled in the art from a reading of thefollowing detailed description of the invention.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided improvedsolid propellant casting tooling consisting essentially of a shaped,filled polymer or copolymer article having at least one outer layer ofcured polyvinyl butyral.

There is also provided a method for modifying solid propellant castingtooling consisting essentially of a shaped, filled polymer or copolymerarticle to provide improved mold release, which consists essentially ofapplying at least one outer coating of polyvinyl butyral to the shapedarticle and curing the same.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The improved solid propellant casting tooling of this invention consistsessentially of a shaped, filled polymer or copolymer article having anouter layer of cured polyvinyl butyral. As mentioned previously, thistooling is used extensively throughout the solid propellant industry tomold or form the solid propellant grain of rocket motors. The polymer iscommonly a fluorinated alkylene polymer or copolymer, such aspolytetrafluoroethylene (PTFE), although other polymeric materials, suchas polyethylene, polypropylene, polyvinyl chloride and the like, whichare rendered porous by the addition of filler materials, may also beused. The filler may be glass fibers, carbon black, synthetic fibers andthe like, including mixtures thereof. The fabrication of such tooling isnot a part of the present invention.

Polyvinyl butyral resins may be prepared from vinyl acetate according tothe following steps: (a) polymerization of vinyl acetate monomer intopolyvinyl acetate; (b) partial hydrolysis of polyvinyl acetate topolyvinyl alcohol; (c) reaction of polyvinyl alcohol withn-butyraldehyde to produce polyvinyl butyral/acetate resin. Polyvinylbutyral resins are commercially available from several commercialsources, such as, Monsanto Chemical Co., Union Carbide Co., etc. Threesuch resins available under the tradename Butvar (Monsanto Chemical Co.)have the compositions (designation/vinyl alcohol content (wt %)/vinylacetate content (wt%)/vinyl butyral content (wt %)/approximate molecularwt): B-72A/19/1.0/80/3500; B-76/11/1.0/88/750; B-98/19/1.0/80/500.

At least one, preferably at least 3, layers of polyvinyl butyral resinare applied to the outer surface of the casting tooling. The resin canbe applied using any conventional technique, such as dipping, spraying,brushing, etc. Sufficient time should be allowed between each layer forthe carrier solvent to substantially completely evaporate. The resincoating is cured by heating to about 100° to 140° F. for about 8 to 48hours. Previously used tooling should be lightly abraded prior toapplication of the resin.

For use with electrostatic-sensitive propellants, the resin coatingand/or the filled polymer or copolymer tooling can be renderedconductive by incorporating therein a conducting ingredient, such ascarbon black.

The polyvinyl butyral coating lacks mold release properties.Accordingly, prior to using the improved tooling of this invention forcasting a propellant grain, a conventional mold release agent is appliedto the outer surface.

The following Examples illustrate the invention. Mold release rods were1-inch diameter, 5-inch long rods of 25% glass-filled Teflon®. Fourcoats of polyvinyl butyral (Butvar B-98, Monsanto Chemical Co.) werebrushed on the rods with one hour between coats to allow the solvent toevaporate. The thus-coated rods were heated at 120° F for 24 hours tocure the resin coat. A top coat of a conventional fluorocarbondispersion (Miller-Stephenson Chemical Co., MS-122) was applied to theresin coated rods.

In the tests which follow, glass-filled Teflon rods, without thepolyvinyl butyral coating, and with and without the fluorocarbondispersion coating, were used for comparison.

The mold release rods were placed in cups and deaerated propellant wasplaced in the cups around the rods. The propellant was cured and therods were removed from the cured propellant using an Instron tensiletesting machine.

EXAMPLE I

The mold release rods were cast into an 88% solids HTPB/Al/AP compositepropellant. The propellant was cured for 15 days at 120° F. A summary ofthe mold release properties is presented in Table I, below:

                  TABLE I                                                         ______________________________________                                                       Shear Stress (psi)                                             Set    Coating(s)    n*       Mean  Std. Dev.                                 ______________________________________                                        1      None          4        32.8  1.3                                       2      1 fluorocarbon                                                                              4        36.5  2.9                                       3      4 polyvinyl butyral                                                                         12       23.5  0.9                                              1 fluorocarbon                                                         ______________________________________                                         *number of samples in set.                                               

Examination of the above data reveals that the uncoated glass-filledTeflon rods in Set 1 released with an average shear stress of 32.8 psi.The addition of one coat of fluorocarbon release agent (set 2) wasineffective, with an average release shear stress of 36.5 psi. Incontrast, the rods coated with 4 coats of polyvinyl butyral and one coatof fluorocarbon release agent (set 3) had an average shear stress of23.5 psi, an improvement of about 28% in mold release properties overthe uncoated glass-filled Teflon rods.

EXAMPLE II

The mold release rods were cast into an 88.1% solids HTPB/AP compositepropellant. The propellant was cured for 10 days at 135° F. A summary ofthe mold release properties is presented in Table II, below:

                  TABLE II                                                        ______________________________________                                                       Shear Stress (psi)                                             Set    Coating(s)    n        Mean  Std. Dev.                                 ______________________________________                                        4      None          4        25.8  1.5                                       5      1 fluorocarbon                                                                              4        26.9  1.1                                       6      4 polyvinyl butyral                                                                         12       18.5  1.9                                              1 fluorocarbon                                                         ______________________________________                                    

Examination of the above data reveals that the rods coated with 4 coatsof polyvinyl butyral and one coat of fluorocarbon release agent (set 6)had an average shear stress of 18.5 psi, an improvement of about 28% inmold release properties over the uncoated glass-filled Teflon rods (set4).

EXAMPLE III

The mold release rods were cast into a 75% solids NEPE/Al/HMX/APcrosslinked double base propellant. The propellant was cured for 5 daysat 120° F. A summary of the mold release properties is presented inTable III, below:

                  TABLE III                                                       ______________________________________                                                       Shear Stress (psi)                                             Set    Coating(s)    n        Mean  Std. Dev.                                 ______________________________________                                        7      None          4        46.5  4.8                                       8      1 fluorocarbon                                                                              4        34.0  2.0                                       9      4 polyvinyl butyral                                                                         12       23.2  1.9                                              1 fluorocarbon                                                         ______________________________________                                    

Examination of the above data reveals that the rods coated with 4 coatsof polyvinyl butyral and one coat of fluorocarbon release agent (set 9)had an average shear stress of 23.2 psi, an improvement of about 50% inmold release properties over the uncoated glass-filled Teflon rods (set7).

EXAMPLE IV

The mold release rods were coated with 2 or 4 coats of polyvinylbutyral, and 1 or 2 coats of a fluorocarbon mold release agent and castinto an 88% solids HTPB/Al/AP composite propellant. The propellant wascured for 15 days at 120° F. A summary of the mold release properties ispresented in Table IV, below:

                  TABLE IV                                                        ______________________________________                                                       Shear Stress (psi)                                             Set    Coating(s)    n        Mean  Std. Dev.                                 ______________________________________                                        10     None          3        38.2  2.7                                       11     1 fluorocarbon                                                                              3        34.7  2.4                                       12     2 fluorocarbon                                                                              3        30.4  2.6                                       13     2 polyvinyl butyral                                                                         9        26.1  0.8                                              1 fluorocarbon                                                         14     2 polyvinyl butyral                                                                         3        23.5  0.3                                              2 fluorocarbon                                                         15     4 polyvinyl butyral                                                                         3        24.2  0.9                                              1 fluorocarbon                                                         16     4 polyvinyl butyral                                                                         9        23.3  0.8                                              2 fluorocarbon                                                         ______________________________________                                    

Examination of the above data reveals that the rods coated with 1 or 2coats of fluorocarbon release agent (sets 11 and 12, respectively)reduced mold release requirements by about 10 and 20%, respectively,over the uncoated glass-filled Teflon rods (set 10). The addition of 2coats of polyvinyl butyral and one coat of fluorocarbon release agent(set 13) provided an improvement of about 30% as compared to theuncoated rods (set 10). The addition of 2 coats of polyvinyl butyral and2 coats of fluorocarbon release agent (set 14) or 4 coats of polyvinylbutyral and 1 or 2 coats of fluorocarbon release agent (sets 15 and 16,respectively), reduced the mold release requirements by about 40%.

EXAMPLE V

The polyvinyl butyral-coated rods from examples I, II and III (sets 3, 6and 9) were cleaned of residual propellant using methyl chloroform. Thepolyvinyl butyral was undisturbed with this cleaning. The fluorocarbonmold release agent was freshly applied. The refurbished rods were castinto the same propellants, cured and the rods removed. A summary of themold release properties is presented in Table V, below:

                  TABLE V                                                         ______________________________________                                                        Shear Stress (psi)                                            Set   Propellant      1st use  2nd use                                                                              3d use                                  ______________________________________                                        3A    HTPB/Al/AP      23.5     25.9   26.3                                    6A    HTPB/AP         18.5     20.0   16.1                                    9A    NEPE/Al/HMX/AP  23.2     22.4   17.8                                    ______________________________________                                    

Examination of the above data reveals that the refurbished rods retaintheir improved mold release properties through 3 complete cycles.

EXAMPLE VI

The mold release rods were coated with two coats of polyvinyl butyralcontaining 7% carbon. This coating had a measured surface resistivity of1.0 ×10⁵ ohms. One coat of fluorocarbon mold release agent was appliedand the rods were cast into a 88% solids HTPB/Al/AP compositepropellant. The propellant was cured for 15 days at 120° F. A summary ofthe mold release properties is presented in Table VI, below:

                  TABLE VI                                                        ______________________________________                                                        Shear Stress (psi)                                            Set   Coating(s)      n       Mean   Std. Dev.                                ______________________________________                                        17    None            3       38.2   2.7                                      18    2 polyvinyl butyral**                                                                         3       21.9   5.3                                            1 fluorocarbon                                                          ______________________________________                                         **with 7% carbon.                                                        

Examination of the above data reveals that the rods coated with 2 coatsof polyvinyl butyral with 7% carbon and one coat of fluorocarbon releaseagent (set 18) had an average shear stress of 21.9 psi, an improvementof about 43% in mold release properties over the uncoated glass-filledTeflon rods.

Various modifications may be made to the invention as described withoutdeparting from the spirit of the invention or the scope of the appendedclaims.

We claim:
 1. Tooling for solid propellant casting consisting essentiallyof a shaped, filled polymer or copolymer article having at least oneouter layer of cured polyvinyl butyral.
 2. The tooling of claim 1wherein said shaped, filled article is polytetrafluoroethylene.
 3. Thetooling of claim 2 wherein said shaped, filled article contains about25% glass filler.
 4. The tooling of claim 2 wherein said polyvinylbutyral layer contains carbon black.
 5. The tooling of claim 4 whereinsaid polyvinyl butyral layer contains about 7% carbon black.